(1) Field of the Invention
The invention relates to the field of semiconductor manufacturing, and more specifically to a method to reduce the dry-etch chamber particle level during the power-down procedure of the dry-etch cleaning process for lithography masks.
(2) Description of the Prior Art
The trend within the semiconductor manufacturing industry to micro-miniaturization and the ability to produce chips with sub-micron features has resulted in a continuous increase of the performance of semiconductor chips while at the same time the cost of semiconductor chips has continued to decrease.
Micro-miniaturization of semiconductor chips has been achieved by significant advances in specific semiconductor fabrication disciplines such as photolithography and dry-etching. Sub-micron features in photo-resist layers have been routinely achieved by the use of more sophisticated exposure cameras, as well as by the use of more sensitive photo-resist materials. The successful transfer of the sub-micron images have been made possible by the development of dry-etching tools and procedures, from an overlying photo-resist layer, to an underlying material that is used in the fabrication of semiconductors. Single wafer etching can now be performed with the tools and procedures used during Reactive Ion Etching (RIE). This allows for individual etching of each single wafer, with end point detection used for only this single wafer. In this manner wafer to wafer uniformity variations, of the layer being patterned using single layer RIE etching, is not as great a problem as previously encountered during batch RIE etching. Large volumes of wafers can therefore be confidently processed using single wafer RIE procedures, with a decreased risk of under or over-etching due to thickness variations of the material being etched.
During conventional dry-etch, the dry-etch can be applied to a stack of thin layers which can include a photo-resist (for patterning the underlying layer) or an anti-reflective coating (ARC, used for covering the surface of an underlying layer and typically formed over an aluminum layer prior to coating the aluminum layer with photoresist). Such etching, however, results in residues or deposits building up on surfaces inside the plasma treatment chamber. Similar buildup of deposits occurs in plasma treatment chambers wherein deposition is carried out.
A requirement of dry-etching procedures is the ability to maintain a strong end point detection signal at the completion of the dry-etch cycle when dry-etching from wafer to wafer. In using single wafer RIE tools, the wafer being etched is moved to the etch chamber of the single wafer RIE tool. The etch chamber contains a window for the monitoring of the etching sequence. Laser endpoint detection apparatus or optical diodes monitor, through this window, the chemistry of the reactants and their by-products. At the conclusion of the etching cycle the chemistry of the by-products has changed, the end-point detection process monitors this change. If however the window through which the endpoint monitoring takes place becomes layered with adhering RIE products, the endpoint detection signal will decrease in intensity which can at times result in erroneous end-point signals.
Residual reaction products adhere to surfaces in the plasma treatment chamber when a film is dry-etched in a chlorine-based plasma or fluorine-based dry-etches. These residues can contain metals (or silicates, depending on which type of dry-etch is used), chlorine, or organics or their compounds. The surfaces that the residues adhere to include upper and lower electrode surfaces, walls of the plasma treatment chamber, clamping surfaces, and any other item that the plasma or its byproducts come in contact with. A build-up of these residues deteriorates the etch performance of the dry-etch. As such, the presence of such residues is undesirable.
Using current one-step cleaning procedures results in a violent transition of the plasma in going from the operating condition of rf-on to rf-off. This results in uncontrolled particle levels within the dry-etch chamber, a condition that is very detrimental to the proper operation and control of the dry-etch process. The present invention addresses this operational aspect of the dry-etch process and teaches a method of significantly reducing the particle count of residues or deposits of residual reaction products in the dry-etch chamber at the end of the dry-etch process.
FIGS. 1a through 1e show the current Prior Art one-step cleaning process as applied to a (photolithography) mask.
The FIG. 1a and FIG. 1b conditions indicated below refer to the repetition, that is a first (FIG. 1a) and a second (FIG. 1b) execution of the same process of cleaning the dry-etch chamber under identical operating conditions of the cleaning operation. A one step mode of the dry-etch chamber cleaning process indicates that the operational conditions during the cleaning process remain constant, most specifically the rf power applied during the cleaning process is not varied.
After the dry-clean process has been applied the first time (FIG. 1a) the dry-etch cleaning sequence is stopped and the conditions inside the dry-etch plasma chamber are allowed to stabilize. The dry-clean operating conditions are then reapplied (FIG. 1b) for a given duration, turned off and, after the conditions in the dry-clean chamber have been stabilized, the dry-clean process is considered completed. The processing conditions for the FIG. 1a and FIG. 1b procedures are identical (since FIG. 1b is, as far as operational conditions is concerned, the same as FIG. 1a) and are as follows:30 mt/600 w ICP/15 w RIE/30 sccm O2/5 min.
These operating conditions indicate that the dry-etch chamber that is being cleaned is of the Inductive Coupled Plasma (ICP) type where an ICP rf power of 600 watts is applied during plasma formation within the chamber, this same power is applied during the dry-etch chamber cleaning procedure while for the cleaning of the dry-etch chamber a RIE etch is applied to the chamber and its contents using 30 sccm of O2 for a time of 5 minutes. The parameter mt stands for milliTorr and is the operating pressure in the dry-etch chamber during processing.
FIG. 1a shows the dry-clean chamber 10, a wall polymer or other residual reaction products 12 and the mask 12 that is positioned on a table 16. A rf coil 18 is provided around and on the outside of the chamber 10, the rf coil 18 generates plasma that attacks the wall polymer 14. The wall polymer 14 evaporates as a consequence of the energy provided by the coil 18 thereby making the cleaning the inside of the dry-etch chamber 10 possible. The wall polymer or other residual reaction products 14 are removed via the suction pump 20. The dry-etch process in the example shown is a dry-etch for a Phase Shift Mask (PSM) with MoSiON etchant and O2 descum. The term descum refers to the process of removing scum whereby scum is an organic or inorganic residue that has collected on the sidewalls of the chamber 10. Using the indicated process, the particle count of the wall polymer or other residual reaction products 12 within the chamber is difficult to control while the particle level is unstable. It is clear that a low particle count is key for the yield of the mask production.
FIG. 1b shows the wall polymer or other residual reaction product molecules 22 being distributed throughout the chamber 10 after the rf coil 18 has been activated at part of the Inductively Coupled Plasma (ICP) process and the RIE etch for the cleaning of the chamber and its contents has been initiated. FIG. 1b represents the power-on, FIG. 1a of the ICP chamber cleaning process.
FIG. 1c shows the condition within the dry-etch chamber between the FIG. 1a and FIG. 1b cycle of the dry-etch process. A number of wall polymer or other residual reaction product modules 24 have settled on the surface of mask 12, a number of other wall polymer or residual reaction product molecules 26 and 28 remain attached to the walls of the dry-etch chamber in a pattern that is not equal on both sides of the chamber but that is determined by complex factors of rf power distribution within the chamber, ionization and polarity of the gas molecules, etc.
FIG. 1d shows the repeat of the dry-etch cleaning process forming FIG. 1b of the dry-etch clean process.
FIG. 1e shows the distribution of the wall polymer or other residual reaction product molecules after FIG. 1b of the dry-etch cleaning process has been completed and after the internal conditions of the chamber have stabilized. It is clear from this that a significant number of the wall polymer or other residual reaction product molecules have been deposited on the surface of the mask 12, which has a negative effect on mask yield.
Noteworthy also is a comparison between entries FIG. 1c and FIG. 1e. This comparison will show that no significant difference has been gained in the distribution of impurity particles within the dry-etch chamber in going from FIG. 1e to FIG. 1c, this despite that fact that the dry-etch process was executed one additional time between these two entries. The repeat execution of the dry-etch process did not result in an improvement of the impurity count within the dry-etch chamber.
U.S. Pat. No. 5,215,619 (Chen et al.) shows a plasma reactor with a cleaning operation.
U.S. Pat. No. 4,786,392 (Kruchowski et al.) shows a fixture for cleaning a plasma etcher.